asolimando opened a new pull request, #21081: URL: https://github.com/apache/datafusion/pull/21081
## Which issue does this PR close? - Part of #20766 ## Rationale for this change After a filter reduces a table from 100 to 10 rows, or a LIMIT 10 caps the output, the NDV (e.g. 80) should not exceed the new row count. Without capping, join cardinality estimation uses an inflated denominator, leading to inaccurate estimates. ## What changes are included in this PR? Cap `distinct_count` at `num_rows` in three places to prevent NDV from exceeding the actual row count: - `max_distinct_count` in join cardinality estimation (`joins/utils.rs`) - `collect_new_statistics` in filter output statistics (`filter.rs`) - `Statistics::with_fetch` (`stats.rs`), which covers `GlobalLimitExec`, `LocalLimitExec`, `SortExec` (with fetch), `CoalescePartitionsExec` (with fetch), and `CoalesceBatchesExec` (with fetch) Note: NDV capping for `AggregateExec` is covered separately in #20926. ## Are these changes tested? - `test_filter_statistics_ndv_capped_at_row_count` - verifies NDV capped at filtered row count - 2 new join cardinality test cases - NDV > rows on both/one side - Updated `test_join_cardinality` expected values for capped NDV - `test_with_fetch_caps_ndv_at_row_count` - verifies NDV capped after LIMIT - `test_with_fetch_ndv_below_row_count_unchanged` - verifies NDV untouched when already below row count - All existing `with_fetch` tests pass ## Are there any user-facing changes? No public API changes. Only internal statistics estimation is affected. Disclaimer: I used AI to assist in the code generation, I have manually reviewed the output and it matches my intention and understanding. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
